1. Field of the Invention
The present invention relates generally to an image processing apparatus for extracting a characteristic value for a transformation of gradation, and more particularly to an image processing apparatus for extracting the characteristic value from an image in which pixel values are rearranged based on magnitudes of the pixel values.
2. Related Background Art
In a case where data obtained from photographing by some photographing apparatus such as a sensor, a camera, etc. are displayed on a monitor screen, an X-ray diagnosis film, etc., it is general that the photographed data are subjected to some gradation transformation, wherein the acquired data are converted into easy-to-observe density values. For example, FIGS. 3A and 3B are views schematically showing a radiation image of the abdomen, wherein the numeral 301 represents a spinal region, the numeral 302 denotes a gas generated in the abdomen, and the numeral 303 indicates a region from which a characteristic value for the gradation transformation is extracted. The characteristic value has hitherto been calculated from pixel values in the region 304, and the gradation transformation has been conducted based on this characteristic value.
There might be, however, a case where a gas assuming a complicated configuration is produced in the abdomen, and this gas region extends over the region for calculating the characteristic value for the gradation transformation. Such being the case, a magnitude of the characteristic value might largely differ depending on a case where the gas extends over the region for calculating the characteristic value and on a case where it does not. A problem arises from this, wherein a scatter in density of the image after the gradation transformation occurs.
What has been described so far is how the prior art is, and it is therefore desired to provide an image processing apparatus capable of stably performing the gradation transformation of an object image with a high accuracy.